It’s not immediately clear how to go about doing what you ask, but I spent a few minutes and there isn’t a very satisfactory solution that seems feasible.
There’s nowhere in the upload record for the caption, and I don’t see an obvious way to go backward from the Upload. . . a possibility might be to put the comment in as the filename and then somehow later pull that back out as the caption. Yeah. That’s about the best you could do, encode that caption as the filename, which would make it show up on mouseover and such. If you wanted, you could then do something else, like get filenames that include a caption and then stick that into the html somehow.